The present invention relates to a system and a method of purifying water. More particularly, the invention relates to a system and a method of purifying water of pathogenic micro-organisms such as Giardia and Cryptosporidium.
Polluted drinking water is a great problem and causes mild affections as well as serious diseases and even deaths. In the US, a number of serious outbreaks have been registered as a consequence of drinking water being polluted. Especially harmful effects are caused by the water-borne parasites Giardia and Cryptosporidium. A crucial problem with these organisms is that they cannot be efficiently eliminated from water by applying known methods. The organisms can be found in the whole world, but are most common in warm countries and they cause diarrhoea, stomach pain and nausea. The most serious outbreak of cryptosporidiosis in the US occurred in 1993 in Milwaukee in which 400,000 people were taken ill with polluted drinking water.
Normally, drinking water is purified in several steps such as by flocculation, sedimentation, filtration through sand and chlorination. However, micro-organisms have an incredible ability to adjust to different environments and they gradually become resistant to common bactericidal systems and disinfectants. Cryptosporidium is, for instance, resistant to chlorine and can resist 3% chlorine for 18 hours. However, the WHO recommend only 0.5 ppm chlorine in drinking water.
In U.S. Pat. No. 5,935,431, an integrated filtration and sterilisation apparatus is described. In the apparatus, inlet water is first filtered through a filter bed of, for instance, active carbon, adsorption resin, zeolite, reduction catalysts, paper, polymers, clay, metals, etc. The filter material in this apparatus serves to separate particulate, organic or inorganic compounds from inlet water. If an inorganic material, such as heavy metals or sulphites, is to be removed, the filter material is preferably an ion-exchange resin, a zeolite or a reduction catalyst. The permeate, i.e. the filtered inlet water, is then exposed to UV radiation and ozone treatment, said ozone treatment giving rise to bacterial killing.
Besides being resistant to chlorine, Cryptosporidium is also very resistant to UV radiation and ozone. 1 ppm ozone for 5 minutes results in only 90% killing. However, such high concentrations of ozone are not permitted; only 0.4 ppm is permitted in outlet water intended for drinking water.
U.S. Pat. No. 5,935,431 does not mention how many ppm ozone are added to the outlet water, but this method suffers from the drawback that in ozone treatment degradation products form from the killed bacteria. It is not advisable to directly use the water discharged from this apparatus as drinking water, on the one hand, owing to the degradation products formed and, on the other, owing to possibly remaining ozone.
Both Giardia and Cryptosporidium have a spore stage in their life cycles. The spores are much smaller than the actual organisms and the oocysts of Cryptosporidium can be as small as 4 xcexcm. Today, only ultrafiltration or reverse osmosis can separate such small particles, but the methods are too expensive to be used for large volumes of water.
Thus there is a great need for a new system for purifying drinking water which is capable of dealing with Cryptosporidium, Giardia and other pathogenic, water-borne micro-organisms.
One object of the present invention is to provide a system and a method in which water containing Cryptosporidium and Giardia can be efficiently purified with excellent results.
Other objects, features and advantages of the present invention will be apparent from the description below.
These objects are achieved by a system and a method according to the independent claims. Particularly preferred embodiments are defined in the dependent claims.
In short, the present invention relates to a system for water purification comprising at least two units, of which the first unit comprises at least one filter unit which consists of a container containing a filter bed of zeolite, and of which the second unit consists of a container enclosing a UV lamp. The two units are connected to each other by means of a conduit for conveying water. According to the invention, the filter bed consists of at least 50% by weight natural zeolite with a pore diameter of 0.05-5 nm. The pressure in the filter bed is 1-4 bar and the UV lamp has a light intensity of 300-500 J/m2.
It has, surprisingly, been found that excellent results are achieved when water containing Cryptosporidium and Giardia is purified in accordance with the method and the purifying system according to the present invention. Preliminary results show that a substantially complete reduction of Cryptosporidium and Giardia is obtained.